Ask Paul: The Cherokee Rattle

Q: I just bought a 1974 300-hp Cherokee Six. It has about 1,200 hours to run, has great oil pressure, good compressions, hardly uses oil, no metal in the filter, but during the run-up at 2,000 rpm, the engine makes quite a deep knock.

If you go from both mags to either the left or right the noise disappears. If the rpm is lower than 2,000 on both mags the noise is gone; if it is above 2,150 rpm the noise is not there.

My engineer is a bit stumped. We have run the engine with no cowls at 2,000 rpm and can’t hear any noises outside. The previous owner of 12 years claims it has always done it, but as a motorcycles engineer, unexplained noises worry me.

JOHN SHAW, Northland, New Zealand

A: John, it’s great to hear from you down in Kiwi land. Having been in your beautiful country many, many times, I’ll give it my best shot to solve the mysterious noise coming from your 1974 Piper Cherokee Six 300. The IO-540-K1A5 engine installed in your aircraft has been a true and reliable workhorse over the years, as you probably know.

From the information you furnished, I don’t think it’s anything serious internally with the engine. The first thing that comes to my mind is this may be another case of the infamous “Cherokee Rattle.” This is the title given to a strange noise that was heard from some of the Cherokee series aircraft since the early days of its introduction into the marketplace. While this phenomenon is rare, it has been with us for many years and, unfortunately, to my knowledge, has never actually been successfully pinpointed. Now don’t let this disturb you, because I’ve got a few things I’d like you to check just to see if we can narrow the search for this strange noise.

I can tell you that both Piper Aircraft and Lycoming spent countless hours and dollars trying to determine what this strange noise was and what was causing it to happen. Most of this research took place in the late 1960s and early 1970s and, as determined as all parties involved were to find the gremlin, nothing definite was ever found, to my knowledge.

In my many years attending the Experimental Aircraft Association’s AirVenture in Oshkosh, I’ve had a few Cherokee owners come to our stand and mention this phenomenon. My response was always the same: “If we went out and flew your aircraft right now, could you induce the noise for me and repeat it at will?” I don’t recall anyone responding with a definite yes. As you know, there is nothing more difficult than trying to diagnose a problem that is inconsistent and this condition has got to be one of the most difficult.

Let me ask a few questions to get us started. Can you actually cause it to happen whenever you want? According to your information, you can hear it on the ground during engine run-up, but when the cowling is removed the noise cannot be heard. That being the case, it may be that the cowl is contacting something at a given rpm, so I’d suggest you do a very close visual inspection of the entire engine installation area, including where the exhaust pipe exits the cowl. I find it interesting that you can cause the noise to disappear by going from one mag to the other. Because the flame propagation is changed slightly within the cylinder when you go from both mags to one mag, this may be causing something in the cowling to react differently.

You didn’t mention whether this noise can be heard during flight. The “Cherokee Rattle” was almost always heard in flight, but couldn’t be induced regardless of how hard one tried.

There are a couple of other things that come to mind, so let’s check those out too. When was the last time the engine mount rubbers were changed? If these have been in service for a long period of time or many flying hours, they may be beginning to allow the engine to sag, causing the cowl to make contact with the engine mount, exhaust pipe, or other components, which may be the cause of the noise.

Another thing that could possibly cause a strange noise is if one of the cylinders has an odd choke in the cylinder barrel. I’ve run across this strange situation a couple of times during my career and it’s one of those conditions that we’d never expect. The condition is caused by the choke in a cylinder barrel coming down the cylinder barrel wall further on one side than the other. It causes the piston to cock slightly at the top of the stroke, which in turn causes the piston skirt to slap the barrel, which we hear as a definite knock.

Things we do to make this appear or disappear are changing the engine rpm, going from both mags to either the left or right mag as you mentioned. Using extreme caution, I’d suggest after you’ve completed your installation inspection and finding nothing there, you run the engine with the top cowl removed and listen to each cylinder with a stethoscope at various rpms right at the cylinder base. If it is a piston slap condition, you’ll be able to hear it using this method.

If the knock is coming from a piston slap condition, I’ve never heard of it causing any serious problems other than being difficult to explain to passengers who may not be comfortable flying in the first place.

John, if all else fails, here’s the information of the current Lycoming representative in your corner of the world, and who happens to be a fellow countryman of yours: Adrian McHardy, P.O. Box 1159, Rotorua 3215, New Zealand; Phone: 64-7-345-3656; Fax: 64-7-345-3657; Email: [email protected]. If you need to contact Adrian, have your specific engine model and serial number, plus total time since new or overhaul.

As you can tell, the condition you are dealing with is not an easy one to figure out, but hopefully you’ll have success after following some of the suggestions offered here.